The invention relates to a phantom.
Electroencephalography, abbreviated EEG, is an important analysis method for characterizing brain activity. To this end, weak electrical currents accompanying brain activity are derived at defined points on the scalp using electrodes. The voltage fluctuations between two of these electrodes are amplified in each case and recorded by a multi-channel recorder as a function of time. The resulting electroencephalogram makes it possible to draw conclusions about brain diseases.
In contrast, magnetoencephalography, abbreviated MEG, is a measurement of the magnetic activity of the brain using external sensors, such as so-called superconducting quantum interference devices (SQUIDs). The magnetic signals in the brain are caused by the electrical currents in active nerve cells. As a result, a magnetoencephalograph can be used to record data that is an expression of the present overall activity of the brain, without time delay.
A magnetoencephalograph provides good spatial resolution and very high temporal resolution. Modern whole-head magnetoencephalographs have a helmet-like configuration, comprising, for example, approximately 300 magnetic field sensors, and this is placed on the head of the patient or test subject without contact during measurement. Since the magnetic signals in the brain amount to only a few femtotesla, outside interference must be shielded to as great an extent as possible.
The key advantages of magnetoencephalography, as compared to electroencephalography, are the easy application of the device, which provides both a large number of channels and precisely known sensor positions and, as a result of the measurement modality, the ability register the activities of deeper brain regions as well.
Recorded brain signals typically constitute a complex composition of many superimposed individual brain activities and also endogenous artifact signals, such as those of the cardiac activity and the eyes and facial muscles. Isolating and localizing the signals associated with the key brain activities from all the sensor signals that are determined, in order to be able to specifically analyze them, is a particular challenge of modern neuroscience.
A difficulty in terms of the localization of electrical currents by way of the magnetic fields measured by a magnetoencephalograph is non-uniqueness in the so-called inverse problem. In this problem, one and the same arbitrarily precisely measured magnetic field distribution may be generated by different arrangements of electrical current densities. Accordingly, limiting assumptions regarding the geometric distribution of the current densities are also required in order to localize the current densities based on the measured magnetic fields. The different back calculation algorithms are based on assumptions.
Different back calculation algorithms are known from Phillips (Phillips, J. W., Leahy, R. M., Mosher, J. C. (1997). MEG-based imaging of focal neuronal current sources. IEEE Transactions on medical imaging, Vol. 16, No. 3, 338-348).
The quality of back calculation algorithms for the localization and quantification of the current densities is ensured using artificially generated current density distributions. For this purpose, electrical or magnetic dipoles having defined intensities are generated in exactly defined local physical sites in a so-called phantom. A phantom, therefore, is a device for generating spatially distributed electromagnetic signals.
A head phantom for this purpose comprises, for example, an array of 32 current dipoles, a computer for controlling a 32-channel dipole driver, and the actual head phantom. Such a phantom is disclosed in the published prior art by Spencer et al. (Spencer, M. E., Leahy, R. M, and Mosher, J. C, 1996. A skull-based multiple dipole phantom for EEG and MEG Studies. Proceedings of the 10th international conference on biomagnetism, Biomag '96, Santa Fe, N. Mex., February 1996).
An electrical dipole is generated in phantoms using a thin coaxial cable, which is actuated via a voltage source. The two contacts at the cable end are open in an electrically conductive medium within the phantom. If the ends are connected to each other by a wire, which is wound into a coil, a magnetic dipole can thus be generated. In this way, each coaxial cable forms a measurement site, and a dipole is generated at this exactly defined point when a voltage is applied. The dipole is sensed by external sensors.
A plurality of dipoles can be generated in the phantom using a corresponding number of spatially distributed coaxial cables, which are actuated electrically via a dipole driver.
A head phantom has a head-shaped configuration, in order to emulate the anatomy of a head. An electrically conductive medium at the interior of the phantom approximately emulates the electrical properties of the brain, in order to allow calibration of the data measured in a test subject on the basis of the data determined with the phantom.
With a phantom, variances in distributions reconstructed by way of the algorithms usually occur with respect to the actual distributions. The lower the variance is between the reconstructed parameters and the actual parameters, such as the intensity and localization of the distributions, the higher is the quality of the overall measurement system. The variances in the data for individual channels can be incorporated in an iterative optimization of the back calculation algorithm.
In order to sustainably improve the quality of the measurement system, the measurement of the current density distributions, or the magnetic field distributions, must be performed with as little interference as possible. To this end, the quality of the shields on the coaxial cables used will be higher or lower, depending on the requirements. Furthermore, in the phantom, it is possible to address only one channel at a time.
The disadvantage of this procedure is, however, that only a single dipole is thus generated at any one time in the phantom. The calibration of the measurement system is consequently comparatively time-consuming and complex problems cannot be simulated rapidly and realistically.
It is known from Friston (Karl J. Friston, 2001. The Neuroscientist, Vol. 7, No. 5, 406-418. SAGE Publications Brain Function, Nonlinear Coupling, and Neuronal Transients) that, in addition to the localization and quantitative description of the neuronal electrical activity, the analysis of interactions between different areas of the brains has also become indispensable in both modern brain research and clinical neurology. Thus, in addition to the type of coupling between the areas (linear or non-linear), a particular crucial modern neurological question is also that of the directionality of the coupling. The directionality of the coupling describes which area of the brain is influenced by the activity of another area that is connected thereto.
Despite progress with respect to the design of the phantoms and the modeling of the measured data, the problem is that the quality of the overall measurement systems described in the prior art remains insufficient for such complex questions, and in particular for the description of the non-linear transfer function of a measurement system.